Impeller, centrifugal compressor, and refrigeration cycle apparatus

ABSTRACT

An impeller according to the present disclosure includes a hub and wings. Each of the wings has a leading edge portion and a body portion. The leading edge portion is positioned on an upper surface side of the hub. The body portion is positioned on a lower surface side of the hub. A tip of the leading edge portion and a tip of the body portion extend from the upper surface side of the hub toward the lower surface side of the hub on a side opposite to a side where the wing is in contact with the hub. In a plan view of the wing seen from a radial direction perpendicular to an axis of the impeller, a profile of the tip of the leading edge portion has a linear shape and a profile of the tip of the body portion has a curved shape.

BACKGROUND

1. Technical Field

The present disclosure relates to an impeller, a centrifugal compressor,and a refrigeration cycle apparatus.

2. Description of the Related Art

Among rotating components used in centrifugal compressors, a componentcalled an impeller applies kinetic energy to a fluid in a manner inwhich the fluid inhaled is accelerated mainly in a direction of thetangent of rotation. The impeller typically has an approximatelytruncated cone shape and rotates about a line connecting the center ofits upper surface having a small diameter and the center of its lowersurface having a large diameter. As disclosed in Colin Osborne et al.“AERODYNAMIC AND MECHANICAL DESIGN OF AN 8:1 PRESSURE RATIO CENTRIFUGALCOMPRESSOR”, NASA CR-134782, April 1975, an impeller has wings (blades)radially arranged.

The leading edge of each wing collides, at an angle, with a fluidinhaled into a centrifugal compressor. The collision makes a differencein velocity between the front surface (suction surface) and back surface(pressure surface) of the wing, applying kinetic energy to the fluid.

In a section from the leading edge of the wing to the trailing edgethereof, an increase in the radius of gyration of the impeller increasesa velocity component of the fluid mainly in the direction of the tangentof rotation. At a position at which the impeller has the maximum outerdiameter, the increase in the velocity component is at its maximum, andthe total amount of the kinetic energy applied to the fluid isdetermined.

In the case where the impeller is designed such that the sectional areathroughout the wing gradually decreases from the leading edge of thewing to the trailing edge thereof, the velocity of the fluid in thedirection parallel to the front surface of the wing can be preventedfrom decreasing.

The velocity of the fluid in the inside (channels between the wings) ofthe impeller, that is, the velocity of the fluid on the front surface ofthe wing depends on a pressure ratio for which a compressor equippedwith the impeller is required. For example, in the case where the fluidto be compressed is air, and in the case of a compressor having apressure ratio of more than 4, the velocity (relative velocity) of thefluid when the fluid is seen from the wing side at the leading edge ofthe wing reaches a transonic speed. A centrifugal compressor whosetarget pressure ratio is 8 is described in Colin Osborne et al.“AERODYNAMIC AND MECHANICAL DESIGN OF AN 8:1 PRESSURE RATIO CENTRIFUGALCOMPRESSOR”, NASA CR-134782, April 1975. In this case, the relativevelocity at the leading edge of each wing is such a high transonic speedas about a Mach number of 1.2.

SUMMARY

The flow of the fluid in channels between the wings of the impeller isvery complex. In a complex flow field, a vortex flow (vortex flow havinga high vorticity of flow) having a low velocity and a high intensity iscreated, and accordingly, an efficient application of kinetic energy tothe fluid from the wings is hindered. In addition, the friction of thefluid in the vortex flow causes a loss. This lowers the pressure ratioand an adiabatic efficiency.

One non-limiting and exemplary embodiment provides a technique forappropriately adjusting the distribution of the velocity of the fluid inthe channels between the wings and for improving the efficiency of thecentrifugal compressor.

In one general aspect, the techniques disclosed here feature an impellerfor a centrifugal compressor including a hub that has an upper surface,a lower surface, and an outer surface, and wings that are fixed to thehub and that are arranged radially around the outer surface of the hub.A wing has a leading edge portion and a body portion. The wing is eachof the wings. The leading edge portion is positioned on an upper surfaceside of the hub. The body portion is positioned on a lower surface sideof the hub. The leading edge portion includes a leading edge. A tip ofthe leading edge portion and a tip of the body portion extend from theupper surface side of the hub toward the lower surface side of the hubon a side opposite to a side where the wing is fixed to the hub. In aplan view of the wing seen from a radial direction perpendicular to anaxis of the impeller, a profile of the tip of the leading edge portionhas a linear shape and a profile of the tip of the body portion has acurved shape.

According to the present disclosure, the distribution of the velocity ofthe fluid in the channels between the wings can be appropriatelyadjusted to improve the efficiency of the centrifugal compressor.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a centrifugal compressor according to anembodiment of the present disclosure;

FIG. 2 is a projection view of a meridional plane of an impeller of thecentrifugal compressor illustrated in FIG. 1;

FIG. 3A is a schematic perspective view of a main wing of the impeller;

FIG. 3B is a partially enlarged side view of the main wing of theimpeller;

FIG. 4 is a graph illustrating the relationship between a wing angle βband a distance from a leading edge;

FIG. 5 is a diagram illustrating the hub-tip ratio of the impeller ofthe centrifugal compressor illustrated in FIG. 1;

FIG. 6 is a configuration diagram of a refrigeration cycle apparatusthat uses the centrifugal compressor illustrated in FIG. 1; and

FIG. 7 is a partially enlarged side view of a main wing of aconventional impeller.

DETAILED DESCRIPTION

The present inventors have analyzed the flow of a fluid (for example,water vapor) in the inside (channels between wings) of an impeller indetail and consequently found that merging and breakdown of large vortexflows result in the production of a region in which a flow is blocked(region in which the velocity of the flow is very low) inside theimpeller. The present inventors have diligently examined the shape of awing that enables the large vortex flows to be inhibited from mergingand breaking down and consequently considered the impeller according tothe present disclosure.

An impeller according to a first aspect of the present disclosure is animpeller for a centrifugal compressor including a hub that has an uppersurface, a lower surface, and an outer surface, and wings that are fixedto the hub and that are arranged radially around the outer surface ofthe hub.

A wing has a leading edge portion and a body portion. The wing is eachof the wings. The leading edge portion is positioned on an upper surfaceside of the hub. The body portion is positioned on a lower surface sideof the hub. The leading edge portion includes a leading edge.

A tip of the leading edge portion and a tip of the body portion extendfrom the upper surface side of the hub toward the lower surface side ofthe hub on a side opposite to a side where the wing is fixed to the hub.

In a plan view of the wing seen from a radial direction perpendicular toan axis of the impeller, a profile of the tip of the leading edgeportion has a linear shape and a profile of the tip of the body portionhas a curved shape.

The impeller according to the first aspect of the present disclosurethat is expressed in another way is an impeller for a centrifugalcompressor including

a hub that has an upper surface, a lower surface, and an outer surface,and

wings that are fixed to the hub and that are arranged radially aroundthe outer surface of the hub.

A wing has a leading edge portion and a body portion. The wing is eachof the wings. The leading edge portion is positioned on an upper surfaceside of the hub. The body portion is positioned on a lower surface sideof the hub. The leading edge portion includes a leading edge. Theleading edge constitutes one edge of the wing in a direction parallel toan axis of the impeller.

A tip of the leading edge portion and a tip of the body portion extendfrom the upper surface side of the hub toward the lower surface side ofthe hub. The tip of the leading edge portion and the tip of the bodyportion constitutes one edge of the wing opposite to the other edge ofthe wing where the wing is fixed to the hub in a radial directionperpendicular to the axis of the impeller.

In a plan view of the wing seen from the radial direction, a profile ofthe tip of the leading edge portion has a linear shape and a profile ofthe tip of the body portion has a curved shape.

In the impeller according to the first aspect, even when the separationof a boundary layer and/or a leakage flow at the wing edge causehigh-intensity vortex flows to be produced in the inside (channelsbetween the wings) of the impeller, the vortex flows can be inhibitedfrom merging and becoming large. In other words, the distribution of thevelocity of the fluid in the channels between the wings can beappropriately adjusted. Consequently, blocking on the inside of theimpeller is inhibited, the fluid flows smoothly, and kinetic energy canbe efficiently applied from the wings to the fluid. In particular,according to the first aspect, the performance of the compressor can bemaintained even under operating conditions of a low Reynolds number anda low specific speed. The use of the impeller according to the firstaspect enables a low-density, high-viscosity fluid (for example, watervapor) to be highly efficiently compressed.

According to a second aspect of the present disclosure, for example,each of the wings of the impeller according to the first aspect has apressure surface and a suction surface. In the plan view of the wingseen from the radial direction, the profile of the tip of the leadingedge portion includes a first upstream portion on a pressure surfaceside and a second upstream portion on a suction surface side, and thefirst upstream portion and the second upstream portion have a linearshape. In the plan view of the wing seen from the radial direction, theprofile of the tip of the body portion includes a first downstreamportion on the pressure surface side and a second downstream portion onthe suction surface side, and the first downstream portion and thesecond downstream portion have a curved shape. With this structure, theeffects in the first aspect can be achieved with certainty.

According to a third aspect of the present disclosure, for example, inthe case where the total length of each of the wings in an axialdirection parallel to the axis of the impeller according to the first orsecond aspect is defined as a meridional plane length in a projectionview of a meridional plane that is obtained in a manner in which thewing is rotationally projected on the meridional plane containing theaxis of the impeller, the leading edge portion occupies a portion of thewing extending from the leading edge to a position 5% of the meridionalplane length away from the leading edge in the axial direction in theprojection view of the meridional plane. The limitation of the range ofthe leading edge portion to a certain degree reduces the likelihood ofthe wings having insufficient length and accordingly enables sufficientenergy to be applied to the fluid.

According to a fourth aspect of the present disclosure, for example, thewings of the impeller according to any one of the first to third aspectsform respective main wings of the impeller. The impeller furtherincludes sub wings. Each of the sub wings is disposed between the mainwings that are adjacent to one another in a circumferential direction ofthe impeller. Considering a throat area (minimum sectional area of thechannels between the wings) that can be calculated from the maximum flowrate for which the centrifugal compressor is required, the sub wings mayhave the sectional shape of the main wings. According to the fourthaspect, a centrifugal compressor having a wider range of the flow ratecan be formed.

According to a fifth aspect of the present disclosure, for example, aratio of the radius of the hub to the radius of each of the wings of theimpeller according to any one of the first to fourth aspects ranges from0.6 to 0.7 at the leading edge of the wing. According to the fifthaspect, the disturbance of the flow field can be effectively inhibited,and the pressure ratio can be increased.

A centrifugal compressor according to a sixth aspect of the presentdisclosure includes the impeller according to any one of the first tofifth aspects and a shroud wall accommodating the impeller. According tothe sixth aspect, a highly efficient centrifugal compressor can beprovided.

A refrigeration cycle apparatus according to a seventh aspect of thepresent disclosure includes the centrifugal compressor according to thesixth aspect. A material whose saturated vapor pressure is a negativepressure at a normal temperature is used as a refrigerant. According tothe seventh aspect, the pressure of the refrigerant can be efficientlyincreased, and accordingly, the efficiency of the refrigeration cycleapparatus can be improved.

According to an eighth aspect of the present disclosure, for example,the material in the refrigeration cycle apparatus according to theseventh aspect contains water. The centrifugal compressor that uses theimpeller according to the present disclosure is suitable for efficientlycompressing a refrigerant containing water (water vapor).

An embodiment of the present disclosure will hereinafter be describedwith reference to the drawings. The present disclosure is not limited tothe embodiment described below.

As illustrated in FIG. 1, a centrifugal compressor 100 according to theembodiment includes a shaft 11, an impeller 2, a back plate 13, and ahousing 15. The impeller 2 is fixed to the shaft 11. The back plate 13is disposed on the back side of the impeller 2. The impeller 2 isaccommodated in the housing 15. The centrifugal compressor 100 is drivenby rotation of the shaft 11 and compresses a working fluid. In thefollowing description, the front surface side of the back plate 13 in adirection (axial direction) parallel to the axis O of the impeller 2 isalso referred to as a front side, and the back surface side thereof inthe direction is also referred to as a back side.

The impeller 2 includes a hub 20, main wings 21 (full blades), and subwings 22 (splitter blades). The hub 20 has an upper surface 20 p havinga small diameter and a lower surface 20 q having a large diameter in theaxial direction, and the diameter of the hub 20 smoothly increases fromthe upper surface 20 p to the lower surface 20 q along the axis O. Themain wings 21 and the sub wings 22 are fixed to the hub 20 and arrangedradially around the outer surface of the hub 20. The main wings 21 andthe sub wings 22 are arranged so as to alternate in the circumferentialdirection of the impeller 2. The sub wings 22 are wings shorter than themain wings 21.

The sub wings 22 are not essential and may be omitted.

The housing 15 has a shroud wall 3, a peripheral member 17, and a frontmember 18. The shroud wall 3 has a shape extending along the impeller 2.The shroud wall 3 protrudes from the impeller 2 toward the front sideand forms an inhalation port 12. The peripheral member 17 forms a scrollchamber 16 around the impeller 2, and the scroll chamber 16 is incommunication with a diffuser formed between the back plate 13 and theshroud wall 3.

FIG. 2 is a projection view of a meridional plane (rotation projectionview) that is obtained in a manner in which the main wings 21, the subwings 22, and the shroud wall 3 are rotationally projected on themeridional plane containing the axis O of the impeller 2. A shapeillustrated on the projection view of the meridional plane is called “ameridional plane shape” in the field of turbomachinery. In theembodiment, the outer circumferential edge of each main wing 21 facingthe inhalation port 12 is defined as the leading edge 31 of the mainwing 21. The outer circumferential edge of each main wing 21 facing theshroud wall 3 is defined as the tip 32 of the main wing 21. Similarly,the outer circumferential edge of each sub wing 22 facing the inhalationport 12 is defined as the leading edge 41 of the sub wing 22. The outercircumferential edge of each sub wing 22 facing the shroud wall 3 isdefined as the tip 42 of the sub wing 22. The leading edges 31 and 41are positioned on the same side in the axial direction as the uppersurface 20 p of the hub 20. In the embodiment, the leading edge 31 ofthe main wing 21 is perpendicular to the axis O of the impeller 2. Thetrailing edge 43 of the sub wing 22 is positioned at the same positionas the trailing edge 33 of the main wing 21. The leading edge 41 of thesub wing 22 is positioned at a position away from the leading edge 31 ofthe main wing 21 toward the rear side. The leading edge 31 constitutesone edge of the main wing 21 in the direction parallel to the axis ofthe impeller 2.

As illustrated in FIG. 3A, each main wing 21 has a leading edge portion24 positioned on the side of the upper surface 20 p of the hub 20 and abody portion 25 positioned on the side of the lower surface 20 q of thehub 20. The body portion 25 is smoothly connected to the leading edgeportion 24. The tip 35 of the leading edge portion 24 and the tip 36 ofthe body portion 25 extend from the side of the upper surface 20 p ofthe hub 20 toward the side of the lower surface 20 q of the hub 20 on aside opposite to a side where the main wing 21 is fixed to the hub 20.As illustrated in FIG. 3B, in a plan view of the main wing 21 seen froma radial direction perpendicular to the axis O of the impeller 2, theprofile of the tip 35 of the leading edge portion 24 has a linear shape,and the profile of the tip 36 of the body portion 25 has a curved shape.A boundary 37 at which the main wing 21 is connected to the hub 20 has acurved shape overall from the leading edge 31 to the trailing edge 33.In FIG. 3B, the axis O extends along the boundary between the leadingedge portion 24 having a linear shape and the body portion 25 having acurved shape. The leading edge portion 24 includes the leading edge 31.The tip 35 of the leading edge portion 24 and the tip 36 of the bodyportion 25 constitute one edge of the main wing 21 opposite to the otheredge of the main wing 21 where the main wing 21 is fixed to the hub 20in the radial direction perpendicular to the axis of the impeller 2.

As illustrated in FIG. 3B, each main wing 21 has a pressure surface 21 pand a suction surface 21 q. The surface of the main wing 21 on therotation direction side of the impeller 2 is the pressure surface 21 p(pressing surface), and the surface of the main wing 21 opposite to thepressure surface 21 p is the suction surface 21 q (non-pressingsurface). Similarly, the surface of each sub wing 22 on the rotationdirection side of the impeller 2 is a pressure surface, and the surfaceof the sub wing 22 opposite to the pressure surface is a suctionsurface.

Impellers disclosed in International Publication No. 2014/073377,International Publication No. 2014/199498, Japanese Unexamined PatentApplication Publication No. 2011-117346, and U.S. Patent ApplicationPublication No. 2008/0229742 are assumed to be used under conditions inwhich the Reynolds number Re becomes about 10⁶. Specifically, acentrifugal compressor as a component of a motor such as a superchargeror a gas turbine that uses air as a working fluid is assumed. TheReynolds number Re is expressed by the following formula (1).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{{Re} = \frac{\rho \cdot R_{{1T}\;} \cdot W_{1\; T}}{v}} & (1)\end{matrix}$

ρ: density of the working fluid (during inhalation)

R_(1T): radius of a shroud at the leading edge of a wing

W_(1T): relative velocity on the shroud side at the leading edge of thewing

υ: kinetic viscosity of the working fluid (during inhalation)

In International Publication No. 2014/073377, International PublicationNo. 2014/199498, Japanese Unexamined Patent Application Publication No.2011-117346, and U.S. Patent Application Publication No. 2008/0229742,the specific speed Ns is assumed so as to be about 0.6 to 0.8. Thespecific speed Ns is an index representing the size of fluid machineryand is expressed by the following formula (2).

Ns=(NQ ^(1/2))/(H⁴)^(1/3)   (2)

N: rotational speed of the axis [rpm]

Q: volume flow rate of the working fluid (entrance) [m³/sec]

H: heat drop (head) [m]

In some centrifugal compressors used in, for example, air-conditioningapparatuses, a compressible fluid other than air is used as the workingfluid. In some cases, a decrease in the viscosity of the working fluiddecreases Re to about 10⁴. These cases have a problem in thathigh-intensity vortex flows are frequently created from the surface ofthe hub and the surface of the wings. Mutual influence between thehigh-intensity vortex flows causes a large disturbance inside theimpeller. Consequently, the performance of the centrifugal compressorsis greatly reduced.

As illustrated in FIG. 7, in a conventional impeller, the profile of thetip 210 a of a wing 210 on the pressure surface side and the profile ofthe tip 210 b of the wing 210 on the suction surface side have a curvedshape overall. Accordingly, a fluid that collides with a leading edge210 c is immediately accelerated. In this case, mutual influence betweenthe high-intensity vortex flows is likely to cause a large disturbanceinside the impeller.

In contrast, in the impeller 2 according to the embodiment, each mainwing 21 has the leading edge portion 24. Since the profile of the tip 35of the leading edge portion 24 has a linear shape, the fluid is unlikelyto be accelerated at the leading edge portion 24. Consequently, theboundary layer is inhibited from expanding, and a position at which alow-energy, high-intensity vortex flow due to the separation of theboundary layer is created shifts to the side that is more downstreamthan in the case of the conventional wing 210 (FIG. 7). Since theposition of the vortex flow shifts to the downstream side, even when alow-energy vortex flow due to the separation of the boundary layer iscreated near the leading edge 31 on a surface (outer surface of the hub20) other than the surfaces of the main wing 21, the positions at whichthe vortex flows are created are different. This inhibits the productionof a region in which a flow is blocked in the inside (channels betweenthe wings) of the impeller 2 due to merging and breakdown of largevortex flows. In other words, the distribution of the velocity of thefluid in the channels between the wings can be appropriately adjusted.This effect is noticeable in a flow field in which the Reynolds numberis a low number of about 10⁴.

The flows are decelerated on the side of the suction surface 21 q of theleading edge portion 24 and accelerated on the side of the pressuresurface 21 p. Since the flows are decelerated on the side of the suctionsurface 21 q, the boundary layer is inhibited from expanding and frombeing separated. Since the flows are accelerated on the pressure surfaceside, a low-energy flow that is created on the side of the suctionsurface 21 q of the adjacent main wing 21 due to the separated boundarylayer can be diverted. The low-energy flow can be prevented from beingmaintained on the pressure surface 21 p and re-collides with the suctionsurface 21 q of the main wing 21 from which the flow has been separated.This inhibits the disturbance of the velocity distribution of the fluidon the pressure surface 21 p of the main wing 21 due to a secondary floworiginated from the adjacent main wing 21 and enables the velocitydistribution to be appropriately adjusted.

As illustrated in FIG. 3B, in a plan view of each main wing 21 seen fromthe radial direction, the profile of the tip 35 of the leading edgeportion 24 includes a first upstream portion 35 a on the side of thepressure surface 21 p and a second upstream portion 35 b on the side ofthe suction surface 21 q. The first upstream portion 35 a and the secondupstream portion 35 b have a linear shape. In the plan view of each mainwing 21 seen from the radial direction, the profile of the tip 36 of thebody portion 25 includes a first downstream portion 36 a on the side ofthe pressure surface 21 p and a second downstream portion 36 b on theside of the suction surface 21 q. The first downstream portion 36 a andthe second downstream portion 36 b have a curved shape. The firstdownstream portion 36 a and the second downstream portion 36 b have acurvature so as to bend into a convex shape toward the side of thesuction surface 21 q. This structure enables the above effects to beachieved with certainty.

As illustrated in FIG. 2, the total length of each main wing 21 in theaxial direction parallel to the axis O of the impeller 2 is defined as ameridional plane length L. The leading edge portion 24 occupies aportion of the wing 21 extending from the leading edge 31 to a position5% of the meridional plane length L away from the leading edge 31 in theaxial direction in the projection view of the meridional plane in FIG.2. The body portion 25 occupies a portion of the main wing 21 extendingfrom the position 5% of the meridional plane length L away from theleading edge 31 to the trailing edge 33. In FIG. 3A and FIG. 3B, theleading edge portion 24 is exaggeratedly illustrated. The limitation ofthe range of the leading edge portion 24 to a certain degree reduces thelikelihood of the wings 21 having insufficient length and accordinglyenables sufficient energy to be applied to the fluid.

As illustrated in FIG. 4, when attention is paid to the wing angle βb ofeach wing (main wing) on the side on which the wing is in contact withthe hub 20, there is no large difference between the wing angle βb ofthe wing (main wing 21) according to the present disclosure and the wingangle βb of a conventional wing between the position (0%) of the leadingedge and the position (100%) of the trailing edge. When attention ispaid to the wing angle βb of each wing (main wing) on the side (shroudside) opposite to the side on which the wing (main wing) is fixed to thehub, there is a large difference between the wing angle βb of the wing(main wing 21) according to the present disclosure and the wing angle βbof the conventional wing. Specifically, since the main wing 21 accordingto the present disclosure has the leading edge portion 24 whose profileof the tip 35 has a linear shape, the absolute value of the wing angleβb is very large between the position (0%) of the leading edge and apredetermined position (5%).

As illustrated in FIG. 5, the impeller 2 according to the embodiment hasa hub-tip ratio (D1/D2) of 0.6 to 0.7. The term “hub-tip ratio” means aratio (D1/D2) of the radius D1 of the hub 20 to the radius D2 of eachmain wing 21 at the leading edge 31 of the main wing 21. In the casewhere the hub-tip ratio is in the above range, the following effects areachieved.

A typically designed impeller of a centrifugal compressor has a hub-tipratio of about 0.4 to 0.5. In the embodiment, in which the hub-tip ratioranges from 0.6 to 0.7, the inflow rate of the fluid entering theimpeller 2 increases, and the pressure ratio is likely to increase.However, the disturbance of the flow field and a reduction inperformance due to the disturbance are likely to manifest themselves.Accordingly, in the case where the main wings 21 having the structuredescribed with reference to FIG. 3A and FIG. 3B are used in the impellerhaving a hub-tip ratio of 0.6 to 0.7, the disturbance of the flow fieldcan be effectively inhibited, and the pressure ratio can be increased.In particular, during high-speed rotation, blocking called inducerchoking near the leading edge 31 of the main wings 21 can be prevented.Consequently, a centrifugal compressor having a high pressure ratio anda wide operating range can be formed.

Embodiment of Refrigeration Cycle Apparatus

As illustrated in FIG. 6, a refrigeration cycle apparatus 200 accordingto the embodiment includes a main circuit 6 through which a refrigerantcirculates, a first circulation path 7 for heat absorption and a secondcirculation path 8 for heat dissipation. The main circuit 6, the firstcirculation path 7, and the second circulation path 8 are filled withthe refrigerant that is a liquid at a normal temperature. Specifically,the refrigerant is a refrigerant whose saturated vapor pressure is anegative pressure at a normal temperature (Japanese IndustrialStandards: 20° C.±15° C./JIS Z8703). Examples of such a refrigerantinclude a refrigerant whose main component is water or alcohol. Duringoperation of the refrigeration cycle apparatus 200, the pressure of theinside of the main circuit 6, the first circulation path 7, and thesecond circulation path 8 is a negative pressure lower than anatmospheric pressure. The term “main component” in the presentdisclosure means the most abundant component at a mass ratio.

The main circuit 6 includes an evaporator 66, a first compressor 61, anintermediate refrigerator 62, a second compressor 63, a condenser 64,and an expansion valve 65. These components are connected in this orderalong a channel.

The evaporator 66 stores a refrigerant liquid and evaporates therefrigerant liquid in the inside thereof. Specifically, the refrigerantliquid stored in the evaporator 66 circulates through the firstcirculation path 7 via a heat exchanger 71 for heat absorption. Forexample, in the case where the refrigeration cycle apparatus 200 is anair-conditioning apparatus that cools the inside of a room, the heatexchanger 71 for heat absorption is installed inside the room andexchanges heat between air inside the room supplied from a fan and therefrigerant liquid to cool the air.

The first compressor 61 and the second compressor 63 compressrefrigerant vapor through two stages. The centrifugal compressor 100described above can be used as the first compressor 61. The secondcompressor 63 may be a displacement-type compressor separated from thefirst compressor 61 or a centrifugal compressor (for example, thecentrifugal compressor 100 described above) connected to the firstcompressor 61 by using the shaft 11. An electric motor 67 that rotatesthe shaft 11 may be disposed between the first compressor 61 and thesecond compressor 63 or outside one of the first and second compressors.Connecting the first compressor 61 and the second compressor 63 by usingthe shaft 11 enables the number of the components of the firstcompressor 61 and the second compressor 63 to be decreased.

The intermediate refrigerator 62 cools the refrigerant vapor dischargedfrom the first compressor 61 before the refrigerant vapor is inhaledinto the second compressor 63. The intermediate refrigerator 62 may be adirect-contact-type heat exchanger or an indirect type heat exchanger.

The condenser 64 condenses the refrigerant vapor in the inside thereofand stores the refrigerant liquid. Specifically, the refrigerant liquidstored in the condenser 64 circulates through the second circulationpath 8 via a heat exchanger 81 for heat dissipation. For example, in thecase where the refrigeration cycle apparatus 200 is an air-conditioningapparatus that cools the inside of a room, the heat exchanger 81 forheat dissipation is installed outside the room and exchanges heatbetween air outside the room supplied from a fan and the refrigerantliquid to heat the air.

However, the refrigeration cycle apparatus 200 is not necessarily anair-conditioning apparatus for cooling only. For example, a first heatexchanger installed inside a room and a second heat exchanger installedoutside the room are connected to the evaporator 66 and the condenser 64with a four-way valve interposed therebetween. This achieves anair-conditioning apparatus that can change its operation between acooling operation and a heating operation. In this case, the first heatexchanger and the second heat exchanger both function as the heatexchanger 71 for heat absorption and the heat exchanger 81 for heatdissipation. The refrigeration cycle apparatus 200 is not necessarily anair-conditioning apparatus and may be, for example, a chiller. A subjectto be cooled by the heat exchanger 71 for heat absorption and a subjectto be heated by the heat exchanger 81 for heat dissipation may be a gasother than air or a liquid.

The expansion valve 65 is an example of a pressure-reducing mechanismthat reduces the pressure of a condensed refrigerant liquid. Thepressure-reducing mechanism may be formed, for example, such that theexpansion valve 65 is not disposed in the main circuit 6, and the liquidsurface of the refrigerant liquid in the evaporator 66 is higher thanthe liquid surface of the refrigerant liquid in the condenser 64.

The evaporator 66 is not necessarily a direct-contact-type heatexchanger and may be an indirect type heat exchanger. In this case, aheating medium cooled in the evaporator 66 circulates through the firstcirculation path 7. Similarly, the condenser 64 is not necessarily adirect-contact-type heat exchanger and may be an indirect type heatexchanger. In this case, a heating medium heated in the condenser 64circulates through the second circulation path 8.

In the case where water is used as the refrigerant in the refrigerationcycle apparatus 200 according to the embodiment, the first compressor 61and the second compressor 63 compress water vapor having a negativepressure. The centrifugal compressor 100 described above is suitable forcompressing a low-density, high-viscosity fluid such as water vapor. Therefrigeration cycle apparatus 200 can operate under conditions in whichthe flow rate of the fluid is small against the required pressure ratio,that is, the Reynolds number is low and the specific speed is low.Accordingly, the centrifugal compressor 100 described above is suitablefor the refrigeration cycle apparatus 200 according to the embodiment.

According to the technique disclosed in the present disclosure, theperformance of the compressor can be maintained even under operatingconditions of a low Reynolds number and a low specific speed. Thetechnique disclosed in the present disclosure is suitable for arefrigeration cycle apparatus that uses a natural refrigerant such aswater vapor. According to the technique disclosed in the presentdisclosure, in particular, the performance of a small-outputrefrigeration cycle apparatus can be improved, and the frequency of themaintenance of the refrigeration cycle apparatus can be decreased.

What is claimed is:
 1. An impeller for a centrifugal compressorcomprising: a hub that has an upper surface, a lower surface, and anouter surface; and wings that are fixed to the hub and that are arrangedradially around the outer surface of the hub, wherein a wing has aleading edge portion and a body portion, the wing being each of thewings, the leading edge portion being positioned on an upper surfaceside of the hub, the body portion being positioned on a lower surfaceside of the hub, the leading edge portion including a leading edge, atip of the leading edge portion and a tip of the body portion extendfrom the upper surface side of the hub toward the lower surface side ofthe hub on a side opposite to a side where the wing is fixed to the hub,and in a plan view of the wing seen from a radial directionperpendicular to an axis of the impeller, a profile of the tip of theleading edge portion has a linear shape and a profile of the tip of thebody portion has a curved shape.
 2. The impeller according to claim 1,wherein each of the wings has a pressure surface and a suction surface,in the plan view of the wing seen from the radial direction, the profileof the tip of the leading edge portion includes a first upstream portionon a pressure surface side and a second upstream portion on a suctionsurface side, and the first upstream portion and the second upstreamportion have a linear shape, and in the plan view of the wing seen fromthe radial direction, the profile of the tip of the body portionincludes a first downstream portion on the pressure surface side and asecond downstream portion on the suction surface side, and the firstdownstream portion and the second downstream portion have a curvedshape.
 3. The impeller according to claim 1, wherein, in a case where atotal length of each of the wings in an axial direction parallel to theaxis of the impeller is defined as a meridional plane length in aprojection view of a meridional plane that is obtained in a manner inwhich the wing is rotationally projected on the meridional planecontaining the axis of the impeller, the leading edge portion occupies aportion of the wing extending from the leading edge to a position 5% ofthe meridional plane length away from the leading edge in the axialdirection in the projection view of the meridional plane.
 4. Theimpeller according to claim 1, wherein the wings form respective mainwings of the impeller, the impeller further includes sub wings, and eachof the sub wings is disposed between the main wings that are adjacent toone another in a circumferential direction of the impeller.
 5. Theimpeller according to claim 1, wherein a ratio of a radius of the hub toa radius of each of the wings ranges from 0.6 to 0.7 at the leading edgeof the wing.
 6. A centrifugal compressor comprising: the impelleraccording to claim 1; and a shroud wall accommodating the impeller.
 7. Arefrigeration cycle apparatus comprising: the centrifugal compressoraccording to claim 6, wherein a material whose saturated vapor pressureis a negative pressure at a normal temperature is used as a refrigerant.8. The refrigeration cycle apparatus according to claim 7, wherein thematerial contains water.